Stabilization of perfluoro oils



Patented jan. 3i, 195

UNITED. STATES PATENT OFFICE the United'istateswof America' as represented by the Unitedetates m m En r l mmi sial No Drawing Applic n iehwary 2' 17. Serial No. 5,894? 7 1 I (o1. wit- 648) This invention relates to the stabilization of perfiuoro oils and more particularly to the treatment by means of fluorine of perfiuoro oils which areunstable in the presenceof' fiuorine'and ot-her vigorous fluorihatihg agents and which may contaih residual hydrogen whereby their stability is improved and the residual hydrogen content thereof is reduced;

Perfiuo-ro oils. are a species of perfiuorocarbons, P'erfiuorocarbons are saturated compoundscom taining only carbon and fluorine. They'are chemically'inert which: makes them particularly useful in applications which involve the handling of flu- ,orine andsi'milar reactivematerials; Such-per-flulorocarbons may be made b'ypassingthe vapors o'fselected hydrocarbons over a bed of cobaltic fluoride. The synthesis of perfluorocarbons is more: fully described in the articlesby Fowler et al'. and

Buriord et al, which respectively begin on pages 1292 and 3190f the March 194-7 issue'of- 'IndustriaP :and Engineering Chemistry (vol. 39,- No. 3).

jPerfiuoro oils are perrl-uorocarhons which are made by fiuorinatio-u of petroleum lubricating oils. Such petroleum lubricating oilsas well as xthe perfiuoro oils mad-etherefrlorn containv molecules having from 14 to: I21 carbon atoms, Per vfluoro oils have average molecular weights varying from .9070 to. 110th Assuming a molecu a weight of 1060, an arbitrary mean ioitntula of (320'542 may be assigned vto these 'penfiuem l-pjlsl is roducedsuch perfiuorocarbohs a e rattan associated with small amduhts, of "in emuletely :fiuorinated products which may be designated ashydrofluorocarbons. In the case 91' a pllQ l il having a sharp boiling point such aspen-fluoreheptane, it is vfeasible to separate it Irom the hydrofluoroheptanes which ma be assflciated therewith by fractional distillation. However, since perfiuoro oils c.ontain molecules having from 14 to 21 carbon atoms they have a boiling range rather than a sharp iboiling :poin-t. Fur

thermore, the boiling :range v of such .petfiuoro :uils overlaps the boiling range ;of the rhydmfiuqrorcarbons that .may be associated therewith. Therefore, it is impossible too'sfil arateva jphl fluqro oil from associated :hydrofiuorocarhohs {by tracitional distillation processes.

It has been found that perfiuorocarhons are not chemically inert unless they :ar rigorously purified. It :was discovered that 'perfiuorocarbons would undergo hydrolysis it treated with dilute caustic under mild conditions unlessthey had- .loeen extensively purified. by .ufractiollal iqh. Samples of perfluoroheptane and perfluorodimethyl cyclohexane which had a boiling range 21' of only a ffew tenths of a. deg-rec. hydrolyzed markedly in comparison with material had the sharp boiling point of anentirely pure compound. The impurity which Qausedrthis re;

53 activity of the: perfluorecarbon was assumed to be: a trace of? incompletely.fluorinated material: containing hydrogen, but this, assumption could not beverified. at the time this discovery was; made due to the lack of a precise analy-ti e1 technique for detecting residual hydrogehin the products at that time.

Perfiuor -Qils finduse as coolants, seal liquids and. lubricants in mechanical devices used tor pumping, storing and disposing of elementary I fluorine; It was found that perfiuoro oilswhich had been extensively treated with fluorinating agents and which were therefor assumed tocontain no residual hydrogen would cons-un ela ee ou s of fiuQrine- Th consum tion of v fluorine eventualiyiresulted destruction of the oil through a breakdown int very lowrhoilir g fragments and carbon. It was later dis coyeregi throu the d vel pment of improv d ah lyt; techniques for detecting ihysiltose perfluoro oils that the oilswhich were unstable the. presence of fluorine had a residual hydrogencontent of the order of (a few tenths Al 11% by wei ht Ho-waver, the marked h mical recetir: ityiof these perfi-uorooils toward fl r ne was ti-neiy out of pro or ion t th r residual were sen c ntent.

In order tha setfiuor oil eau es? to e u in dev c ta haud ihesleme ta the nine, i hec ssa -y to is a g sthe 9 sic il Zie t ese ils tha 2.1. Wll ctive wi h-fluorine a temntstesta perflu ro is b r peated y treat ol d cobalt flu ride a :29. 9 2,59" .C- c u e s ul- 9. 7 a e?- r sti i react re {to ,fiu ihee he ab w e i eapt vea q hfiuq e ils a hadlro wt em d L acte with qi h ii if l had a -1 =1 3 %l l u fitness? 9 i tha had n t be n s tr ate h t he tie eu1 n t whol y stat sti al-since they I .v 0 jqn usable a te alper od Q t ol qr ih i was di c vere he herfiuo o 9. i h had ee treated with fluorine at a11y xfigged te e al sti fairl ircect e to fl shesit the e ore, bi t q th s t in.- venti t d vise im thc o stab fiu o 01. 5.8 thetisuqhpil wi act he au with l Q ll hlk 9 .116 -V 9 TQ 5 ,fi inaiiee a en A fu the ab ect 0f th pretes as tiorl is 'to treat perfluoro oils so as to remove therefrom any residual impurities, such as hydrogen which may be causing said oils to be reactive with fluorine. A still further object is to produce fluorinated lubricating oils that ar not unstable in the presence of fluorine and other vigorous fluorinating agents. Another object is to produce stable perfluoro oils by a simple and eificacious procedure. Other objects will appear hereinafter.

These objects are accomplished in accordance with the present invention in which perfluorocarbon materials, such as fluorinated lubricating oils having an approximate formula, 0201 42, are rendered relatively inert to elementary fluorine by treatment with the latter material under controlled conditions. The treatment employs fluorine in concentrations up to 50%, but preferably about 20% mixed with a suitable diluent, such as dry nitrogen. This mixture of gases is bubbled through the liquid perfluoro oil at a suitable rate in a simple tubular reactor which may be heated and may or may notbe equipped for stirring. Reactive anhydrous catalysts such as cobaltic fluoride (COFs), manganese trifluoride (MnFs), or ceric fluoride (CeF4) may be present, if desired, in amounts of the order of 2% to by weight of the perfluoro oil. The essential feature of the treatment is to complete the reaction between the fluorinated oil and the fluorine at each of several temperatures in an increasing scale beginning at a relatively low value in the range from 60 to 150 C. This may be accomplished by treating with fluorine at a fixed temperature until analysis of the gases leaving the reactor shows that very little fluorine is being absorbed, after which the temperature is slightly increased and treatment with fluorine continued until analysis of the exit gases again shows that little fluorine is being absorbed in the reactor, and so on. Since the absorption of fluorine is temperature dependent, the following alternative procedure is also expedient. The exit gases from the reactor may be analysed at frequent intervals, and the temperature slowly raised at treatment with fluorine proceeds in such a manner as to maintain the percent of fluorine absorbed at any convenient value less than 100%, such as 80-95%. This alternative procedure is somewhat more convenient in operation and results in equally good stabilization but with a slightly reduced yield. In accordance with another modified procedure, the temperature throughout the fluorine treatment is maintained high enough so that essentially all of the fluorine is being absorbed by the fluorinated lubricating oil, and the concentration of fluorine in the exit gases approximates to parts per million. Fluorine treatment and the concomitant upward adjustment of temperature is continued until as high a temperature of treatment as possible is reached. This is usually determined by the boiling point of the liquid being treated, and in the case of perfluoro oils having an approximate formula, C20F42, this terminal temperature is about 300 C. The treatment is considered complete when the fluorine absorption at the highest temperature of treatment is of the order of 3% to, 6% of the fluorine being fed to the reactor.

Fluorinated lubricating oils which have been so treated absorb very little fluorine at temperatures below their final treatment temperature and have a low hydrogen content (less than 0.001% by weight). They may be used in contact with fluorine and similar corrosive gases with very much less cracking and degradation than is possible with fluorinated lubricating oils that have not been subjected to the stabilizing treatments of the present invention. Available test methods indicate that perfluoro oils which have undergone stabilization by this method are many times more stable to the action of elementary fluorine and similar fluorinating agents. For example, a perfluoro oil having a very low hydrogen content which has not undergone stabilization will absorb all fluorine supplied to it at temperatures as low as 200 C., whereas the same oil, when stabilized in accordance with the present invention, will absorb only from 3% to 7% of the fluorine supplied to it at 300 C.

In order that the invention may be more fully understood the following detailed examples are given to illustrate it. It is to be understood that these examples merely illustrate but do not limit the present invention.

EXAMPLE I 700 parts by volume of a perfluoro oil containing 0.073% of hydrogen were placed in a copper cylinder which served as a reactor. This reactor was provided with an electrical heater, a stirrer and a fluorine inlet at the bottom which was covered by a 100-mesh copper screen. Fluorine diluted with nitrogen to a fluorine concentration of 20% was passed into the reactor at a rate of 1000 parts by volume per minute. The temperature of the reactor was maintained at 150 C. during the first 2 hours of treatment and then raised to 200 C. for the next 4.4 hours of treatment. During the next period of 3 hours the temperature was held at 250 C., and finally the temperature was maintained at 300 C. for the last period of treatment for 3 hours. A 78% yield of a stabilized perfluoro oil was obtained which showed very little tendency to absorb fluorine at 300 C. The stabilized oil had a lighter color than the oil which had not been stabilized by the treatment with fluorine.

The unstabilized perfluoro oil mentioned in the preceding paragraph could not be successfully stabilized by treating it at a constant fixed temperature of 150 C. with 20% or 40% fluorine supplied ata flow rate of 5 units of volume of gas per minute to each 3 units of volume of oil in the reactor. Even after from four to six hours of treatment with fluorine, the treated oil retained the property of absorbing over 80% of the fluorine supplied thereto at 250 C. This showed that stabilization of this oil cannot be satisfactorily accomplished by treating it with fluorine at a constant fixed temperature.

EXAMPLE II Unstabilized perfluoro oil which contained less than 0.06 of hydrogen was treated with fluorine in a copper cylinder which was electrically heated and equipped with a stirrer and to which fluorine was admitted through a mesh copper screen "'11 jiio se areentie ehe 2e in an wieiae.

age yieikl' en nese seven runs "was 36% "with when or 80% a'r'idf'a'high yie1uef'90% The 'odue'tjhad eteryiqw degree or ability to b}- sorbfluorine compared with 'tfieniista'bilized oil gram which it wasbr'enar'ed. The "stab'ilizi'ed'oil osit'y ;;58; s'c'onds saybem (spout-i9 I at 2m" F.-, whereas theunstabili'zed from which i't 'wa" pre are man a 'visco'sitS'! j 65 .8"se cond s Saybblt about'Z B centiizoifsesat 210 F. The unstabili'ze'd on-naq enux boiling iib'int at a pr ssure 'of 1 0' mm. or 155 fc., whereas e stabilized on made n eceordame with jthi's more hail a demo; boiling point of 140 "C. at pressure of mm. Theyiems in'this example 'f better than 'in the preced ng example beas I or slight i'm 'r'o'vement's made in the eransane and'op'eratifig technique.

700 parts by volume of a perfiuorb'oll eomain in'g "0-i073% of hydrogen were piaeea in a reactor Sflch as that described in Exam le I. TO the Oil was added anhydrous c'obaltous fluoride (COFz) man amount equitalen't to 5% or the weight or menu, The readtor wasbrought to a temperstore of 150 C. "and held. there for two hours whileth'e, reactants werefstirredfand fiuo fine was passed into the reactor'at'a rate of 1000 sens by v lume per minute. Their the temperature or the reactor was inr'e'ased to 200 elhd held there for four hours while stirring' and the introduction or fluorine were polltimled The temperature was then raisegyto 250' C., and held there forthre'e hours and finally raiseq to 300 C. and held therefor tnreejno rs while eonon one, thefintroduotion or 20 nuonne to "the resistor at therate of IOOO'iJarts by "volume per minute. I A'90% yielclof "a stabilized'iierfluoro oil which ha da very slight tendency to absorb fluor ne "at 300 J. was obtained. Much 'of'theco baltous fluoride in the reactor was 'converteId to eeb'eitic 'fluoride by reatio'nwith fluorine, The reactor used in'this example and the 'p'erfluoro oil treated were the same as in Example I, The conditions of treatment were substantially 'the 'eszeept for the em oyment of 'Cbbalt fliiofine as a batalyst, It'will be s en by omparing thisfexample with Example I 'thatthe useof cobalt nuoride as eeetny t resu ts "m some what better yield than was obtained when "no catalyst was present.

IETXAMPIJFJ IV I number of treatments ofjan unstenmzed per fiuo'ro oil which contained less than 0.06% of resia'u'al hydrogen were Carried out by su lying fluorine thereto under the conditions indicated 60 i'n"the"f01lowi'ng table. In'th'e following table Fluorine new rate designates the number of uiii'ts of Volume of diluted, fluorine gas passed into annitfVolur'ne of ip'erfluoro oil per minute. lu rin 'ebheemrati n" j gives the "percent by volume of fluorine infth'e fluorine d'ry nitrogen mixtures. The Time infhburs indicatesthe number of hours the p'erfluoro oil "was subjected to fluorine treatment. In all ten hour runs the oilwas 'treatedwith fluorine at 150"Cfifor three ours, at 200 C. rer two h Our S, their at 250' C. for two hours,'and finally at 280 C. for three hours, men five hour runs the time of treat me "at eaoh temperature was halved. "In e'ach I the temperament the termination or the 0 treatment was 280 G. Under "Cata1yst the per cent indicates the ratio of the weight of catalyst to the weight of perfluoro oil being treated.

one of "the alternate *i'nethocis f stdoiliiing pei'fiuofo oils in meme-nee withthe nr'esent irrventionis to start the fluoifmetreatmentfl about 150 C.,' and-to maintainaoon'staht absorptionof fluorine by variation of'the'treatment temperature, Thusa gradual increase of temperature nermitsthe maintenance of' fa fixed rate of absorption of fluorine. rhe'rouowmg table gives the data one number of stabilization made on a 'pei'fiuo'ro oil having ar sidual hydrogen contentof 0.1% and omposed ofhidlec'ules having ane er ee or 20 carbonatonis. 'These runs wer' conductedso tIIat 's ibstahtiaIW lOOFZ, or the 45 a he was absorbed ep't mtne final part or the reatmem, The 'Ii's'w'eie'eachstarted-at a temperature mgh 'enougn to cause 100% aresorip'tion 'of fluorin the 100% absorption temperature was found at once, and the temperature of 'treatmefitslowly m'cie'asea to maintain [1:007); fluorine absorption until the maxtemperature (usually "300" 'C.) was reached. After reaching the maximum temperature, the fluorine absorption decreased slowly with continued treatment The runwas terminated when the -fluorine absorption had become 1 0% or less as shown by analysisof the exit gases rrom' the reaotor. The -100 absor-ntion was followed and maintained by testing the eitheust gases with moist potassium-iodidepaper. I I II I I I I I I the following table f fFuorine new rate designates thenumber oi imits of volume o'f -cli'luted fluorine gets passed into a unit volume of perg-5 fluoro oil in the reactor per -minute. The Time in hours designates the number of hours the perfiuoro oil is subjected to fluorine treatment. The Termination temperature" indicates the highest "temperature to which' the perfluoro oil #1 wa's'l-ieated before'the-fluorination *treatment was ended. Unqer Catalyst the per *oerit indicates the "am of the'we i-g-ht or oatalystto the weight o r' per'fiuere on "being tiiitegl. "Fluorine eoncentration" gives the per cent-myvo umenrnuo- 75 fine in nuorme mtrdg en g as Table 2-1 Fluorine Fluorine Termina- Crude Run Ooncen- Flow Time tion Tem- Catalyst Yield tration Rate perature Per cent .Hours. C. Per cent 18 1. 7 6. 75 81 37 1. 7 5.- 75 87 37' 1.7 5. 5- 87 18 3.3 5.52 300 "410..--.. 81. 18 1. 7 13. 5 250 3.2% CoFz/ 87 18 1.7 6. 75 300 do 81 18 6.7- 3 3 None. 81

In all of the above runs it was noted that for each 300 cubic centimeters of perfluoro oil being treated from l.2 to 1.? mols of fluorine were absorbed. The runs with 18% fluorine required the longest treatment time and, while the yields of stabilized oil were pooien'the product had a somewhat better stability toward fluorine than Droducts'secured in s'omeof the other runs. The one run (03-4) made with 18% fluorine at twice the normal flow rate gave a better yield of a product of very slightly inferior stability to fluorine than the products made at the normal flow rate. The runs with 37% fluorine were made in somewhat less than the total time required for treatment with 18% fluorine at the 1.7 flow rate and resulted in greater yields of products which were but slightly less stable to fluorine than the products stabilized with 18% fluorine at the 1.7 flow rate. All of the runs described in the above table were carried out in a small reactor which consistently gave smaller yields than the'rcactors used in obtaining .the results which were described in the other examples or this application. I

Attempts to stabilize the same perfluoro oil used' in the runs recorded in Table 2 above by' treating it for about 6 hours at aflxed temperature of 150 C. with or 40% fluorine passed. into the oil at flow rate of 1.7 unit volumes of gas per unit volume of oil per minute resulted in an unstabili'zed product which absorbed'more than two thirds of the fluorine supplied thereto at 250 C. Two more attempts were made to stabilizethis same perfluoro oil by treating it for about 12 hours at a'flszed temperature of'300" C. with 120% and 40% fluorine. After 12 hours of treat ment with 20% fluorine a crude yield of only" 52% was obtained, and'the product was unsatis factorily stabilized since it absorbedof the fluorine suppliediheret'o at 250 C. After 14' hours of treatment with fluorine almost no perfluoro oil was left in the reactor. tempts to stabilizeperfluoro oils at single fixed tem eratures showed that, at moderate tempera-' tures treatment with fluorine yielded products which were essentially unstabilized and capable of readily absorbing much more fluorine at higher lated on a basis of thejratio of the amountof specification grade oil. charged to thereactor to. the amount of specification grade oil recovered from the reactor after the, treatment is com-, plete. The yields using perfluoro oils containing 0.1% or less of residual hydrogen generally vary from 80% to 95%. I V

The preferredapparatus used for treating perfluoro oils with fluorine comprises acopper-or monel tube equipped with a heating jacket, thermocouple well and a motor driven, paddle-shaped stirrer. At the bottom ofthe tube where the fluorine enters a mesh bronze screen is placed across the interior opening. This screen dis; perses the entering fluorine to prevent any effect similar to ignitionat a jet. In operation, the unit is 'filled nearly full of perfluoro oil to be treated, and diluted fluorine gas is bubbled through it at a suitable flow rate. The exit gases from the reactor unit pass through a reflux con denser or mist trap and thence to a gas sampling manifold-before being vented to a disposal unit.

The fluorine concentration in the exit gases was conveniently followed by an analysis based upon the liberation of iodine from a potassium iodide solution followed by titration with a standard thiosulfate solution. The gas was Sam pled in calibrated glass bulbs, after which fluorine was absorbed by admitting a potassium iodide solution.

From the data obtained it appears that stabilination of a perfluoro oil with fluorine is promoted by the following factors: (1) intimate contact between the fluorine and perfluoro oil achieved by rapid and highly eflicient stirring; (2) high flow rate of gas; (3) moderate fluorine concen tration in the gas, and (4) the presence of catalytic proportions of cobalt fluoride in the reactor. Careful attention to these factors results in better yields, higher stability characteris-l tics of the products, and completion of the re-I action in the shortest possible period of time. It was also noted that better yields were obtained if the fluorine absorption was near 100% throughout the run resulting in the shortest possible time of treatment, and if the temperature was smoothly increased to the highest possible value, 285-315 0., which was determined by the boiling point of the oil and the characteristics of the reactor. If the temperature was too high, particularly during the early stages of the run the -yield was decreased, and if the run were extended by holdingthe temperature too low, the yield fell off due to cracking.

It has been found that stirring greatly facili-' tates the'absorption of fluorine by unstabilized perfluoro oils. In one reactor it was found that at C. the fluorine absorption was 33% when the stirrer stood idle but that 83% of the fluorine'being passed into the oil was absorbed when the stirrer was operated at 1200 R. P. M.- In another reactor it was noted that at 100 C. only 0.5% of the fluorine was being absorbed when the stirrer stood idle but that 61% of the fluorine was absorbed when the stirrer was rotated at 600 R. P. M. and 83% of the fluorine was being absorbed when the stirrer was rotated at 1200 R. P. M.

The optimum flow rate for fluorine depends both upon the design'of the treatment apparatus and upon the concentration of the fluorine. For a given treatment system, the optimum con centration' of fiuorine'and flow rate must be experimentally determined to give the best possible yield in the' shortest treatment time. Obviously, a high flow rate of fluorine which causes excessive losses of perfluo-ro oil from the treatment system due to misting is to be avoided. If the fluorine concentration is held down to about 20% the flow rate may be as high as is feasible provided no perfluoro oil is being entrained from the reactor in the form of mist. Flow rates varying from .1 to 7 units of volume of diluted fluorine gas per.

unit of volume of perfluoro oil in the reactor per minute have been found to be feasible. However, flow rates of about 3 to 4 units of volume of diluted fluorine gas per unit of volume of perfluoro oil per minute appear to be optimum. Flow rates greater than this are inadvisable with 37% fluorine if degradation of the oil is to be avoided.

The fluorine used in the reaction is diluted with an inert gas such as nitrogen to cut down the vigor of the ensuing reaction. Fluorine concentrations varying from 5% to 50% may be employed, but, as indicated in the preceding paragraph, it is inadvisable to employ high fluorine concentrations when the diluted fluorine gas is flowing into the reactor at a high flow rate, if degradation of the perfluoro oil being treated is to be avoided. In many reactors it was found that a fluorine concentration of about 20% gave the best yields.

It was found that the use of catalytic proportions-Z to perfluoro oil being treatedof cobalt fluoride tended to increase the yield and quality of the stabilized perfluoro oil produced. In one set of reactors the best yield obtained without the use of cobaltic fluoride was 88% compared to 93% with catalyst present, while 98% of the recovered charge boiled in the proper range when cobaltic fluoride was used, compared to 90 to 96% without a catalyst. The cobalt fluoride is added to the reactor in the form of anhydrous cobaltous fluoride and, is converted in situ to cobaltic fluoride by reaction with fluorine. Manganic fluoride (MnFs) or ceric fluoride (Cel a) may be used in place of cobaltic fluoride as a catalyst.

Manganese fluoride is added to the reactor as anhydrous manganous fluoride (MnFz) and converted in situ to manganic fluoride by fluorine. Similarly, anhydrous cerous fluoride (CeFs) is added to the reactor and converted in situ to ceric fluoride.

Stabilized perfluoro oil from a physical point of view is practically identical with unstabilized perfluoro oil except for the removal of any color present in the starting material. The identical distillation fractions before and after stabilization are surprisingly alike. Since variations in physical properties are minor, the stabilization process herein described can be readily adapted to perfluoro oils meeting certain desirable physical specifications and will not cause changes which result in a large proportion of the stabilized perfluoro oil having properties which lie outside of the desirable specifications of the untreated perfluoro oil.

The chief chemical reaction which takes place during the stabilization treatment appears to be the replacement of residual hydrogen with fluorine. However, it is entirely possible that other reactions such as removal of double bonds and isomerization or other changes in molecular configuration lead to the formation of molecules that are no longer reactive with fluorine.

As can be seen, in accordance with the present invention, a method has been developed that leads to stabilization of perfluoro oils toward fluorine gas and other vigorous fluorinating agents. This method comprises treatment of the perfluoro oil with dilute fluorine at slowly increasing temperatures up to the boiling pointo'f the oil. Such treated perfluoro Oils show a marked stability toward elementary fluorine and will react only slightly at elevated temperatures based upon the weight of 10 in the neighborhood of 300 C. Yields are good averaging 88% or better. The removal of hydrogen appears to accompany the process of stabilization and may be the basis of this phenomenon. Many of the untreated oils which contain approximately 0.02% of residual hydrogen are found after the stabilization treatment to contain 0.001% or less of residual hydrogen.

Resort may be had to such modifications and variations as fall within the spirit of the invention and the scope of the appended claims.

We claim:

1. The process of stabilizing and removing chemically combined residual hydrogen from a perfluoro oil containing less than 1% of residual hydrogen which comprises subjecting said perfluoro oil to treatment with fluorine at a series of increasing temperatures from a base starting range of 60 to C. to a maximum temperature lying between 250 C. and 315 C.

2. A process as recited in claim 1 which is carried out in the presence of an anhydrous catalyst selected from the group consisting of cobaltic fluoride, ceric fluoride and manganic fluoride.

3. The process of stabilizing and removing chemically combined residual hydrogen from a perfluoro oil containing less than 1% of residual hydrogen which comprises passing diluted fluorine gas though said perfluoro oil at a base starting temperature lying in the range of 60 to 150 0., increasing the temperature by an increment of several degrees centigrade when it is found that little fluorine is being absorbed by the perfluoro oil, making at least one more further stepwise increase in the temperature of treatment when it is again found that the perfluoro oil is absorbing little fluorine, said stepwise increase in the temperature of treatment being continued until final treatment temperature lying between 250 C. and 315 C. has been attained, and continuing the treatment with diluted fluorine at the final treatment temperature until very little fluorine is being absorbed by the perfluoro oil.

4. The process of stabilizing and removing chemically combined residual hydrogen from a perfluoro oil containing less than 1% of residual hydrogen which comprises passing diluted fluorine gas through said perfluoro oil at a base starting temperature lying in the range of 60 to 150 C., gradually increasing the temperature so that the fluorine absorption is substantially continuously complete, terminating the treatment with diluted fluorine at a temperature lying between 250 C. and 315 C. when the perfluoro oil is absorbing 10% or less ofthe fluorine supplied thereto at the final treatment temperature.

WILLIAM B. BURFORD, III. CLIFFORD E. WEBER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PA'I'ENI'S OTHER REFERENCES Struve et al., Ind. Eng. Chem., vol. 39, 352-4 (1947). 1 

1. THE PROCESS OF STABILIZING AND REMOVING CHEMICALLY COMBINED RESIDUAL HYDROGEN FROM A PERFLUORO OIL CONTAINS LESS THAN 1% OF RESIDUAL HYDROGEN WHICH COMPRISES SUBJECTING SAID PERFLUORO OIL TREATMENT WITH FLUORINE AT A SERIES OF INCREASING TEMPERATURES FROM A BASE STARTING RANGE OF 60* TO 150*C. TO A MAXIMUM TEMPERATURE LYING BETWEEN 250*C. AND 315*C. 